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Supercapacitors Can Replace a Backup Battery for
Power Ride-Through Applications − Design Note 450
Jim Drew
09/08/450
Figure 1. 5V Ride-Through Application Circuit Delivers 20W for 1.42 seconds
Introduction
Supercapacitors (or ultracapacitors) are fi nding their way
into an increasing number of applications for short-term
energy storage and applications that require intermittent
high energy pulses. One such application is a power ride-
through circuit, in which a backup energy source cuts
in and powers the load if the main power supply fails
for a short time. This type of application has typically
been dominated by batteries, but electric double layer
capacitors (EDLCs) are fast making inroads as their
price-per-farad, size and effective series resistance per
capacitance (ESR/C) continue to decrease.
Figure 1 shows a 5V power ride-through application
where two series-connected 10F, 2.7V supercapacitors
charged to 4.8V can support 20W for over a second. The
LTC3225, a new charge-pump-based supercapacitor
charger, is used to charge the supercapacitors at 150mA
and maintain cell balancing while the LTC4412 provides
automatic switchover between the supercapacitor and
the main supply. The LTM4616 dual output DC/DC
μModule
TM
regulator creates the 1.8V and 1.2V outputs.
With a 20W load, the output voltages remain in regulation
for 1.42 seconds after the main power is removed.
Supercapacitor Characteristics
A 10F, 2.7V supercapacitor is available in a 10mm ×
30mm 2-terminal radial can with an ESR of 25mΩ. One
advantage supercapacitors offer over batteries is their
long lifetime. A capacitor’s cycle life is quoted as greater
than 500,000 cycles, whereas batteries are specifi ed for
only a few hundred cycles. This makes the supercapaci-
tor an ideal “set and forget” device, requiring little or no
maintenance.
Two critical parameters of a supercapacitor in any ap-
plication are cell voltage and initial leakage current. Initial
leakage current is really dielectric absorption current,
which disappears after some time. The manufacturers
of supercapacitors rate their leakage current after 100
hours of applied voltage while the initial leakage current
in those fi rst 100 hours may be as much as 50 times the
specifi ed leakage current.
The voltage across the capacitor has a signifi cant effect
on its operating life. When used in series, the superca-
pacitors must have balanced cell voltages to prevent
overcharging of one of the series capacitors. Passive cell
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
μModule is a trademark of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
V
IN
C
+
C
–
SHDN
V
SEL
C
OUT
CX
GND
PROG
LTC3225
C2
10F
2.7V
C3
10F
2.7V
R2
470k
Q1
Si4421DY
Q2
Si4421DY
R1
12k
C1
1μF
C4
2.2μF
5V
C5
22μF
V
IN
GND
CTL
SENSE
GATE
STAT
LTC4412
R3
4.78k
R4
10k
V
IN1
V
IN2
GND
LTM4616
C6
100μF
C8
100μF
C7
100μF
V
OUT1
FB1
ITHM1
V
OUT2
FB2
ITHM2
V
OUT1
=
I
O1
= 7A
1.8V
V
OUT2
=
I
O2
= 6A
1.2V
